Journal of Earth Sciences and Geotechnical Engineering, vol. 4, no. 2, 2014, 1-21
ISSN: 1792-9040 (print), 1792-9660 (online)
Scienpress Ltd, 2014
Dynamics of Dust Clouds Produced by Off-road Vehicle
Driving
Dirk Goossens 1 and Brenda Buck 2
Abstract
Field experiments were conducted in the Nellis Dunes Recreational Area (Nevada, USA) to
study the dynamics of dust clouds produced by off-road vehicle (ORV) driving. Tests were
performed with a 4-wheeler (quad), for 3 driving speeds between 24 and 40 km h-1 and for 7
different types of surfaces. Fine as well as coarse-grained surfaces were investigated in the
tests. Results show that concentration of PM10 (particles <10 μm, which are the most easily
inhaled) behind an ORV can be as high as 160 mg m-3 or even more at nose level of quad
drivers and passive recreants standing near an ORV trail. ORV generated dust clouds are
characterized by a main wave of high concentration followed by an exponential decrease
with irregularities (secondary waves) until background concentration has again been
reached. Peak concentrations usually occur between 5 and 7 seconds after the passage of an
ORV and are higher for silty surfaces than for sandy surfaces. Under no-wind and
headwind conditions it takes at least one minute before PM10 concentration has returned to
the background level. For tailwind the duration of elevated concentration is of the order of
30 seconds. For winds blowing obliquely to the driving path it is only a few seconds.
Elevated PM10 concentrations may occur up to several hundreds of meters behind an ORV,
even if no dust is visually noticeable. In-echelon driving is thus strongly dissuaded unless
appropriate protection is worn against inhaling the dust.
Keywords: Dust, PM10, Off-road driving, Quad, Concentration, Health
1 Introduction
Off-road vehicle (ORV) driving is one of the most prevalent and fastest growing leisure
activities on public lands worldwide [1],[2]. For example, in western Australia the sales of
off-road motorcycles and quad bikes (four-wheelers) increased by 67% between 2004 and
1
Department of Geosciences, University of Nevada Las Vegas, 4505 S Maryland Parkway, Las
Vegas, NV 89154-4010, USA. Also at: Division of Geography, KU Leuven Department of Earth and
Environmental Sciences, Geo-Institute, Celestijnenlaan 200E, 3001 Leuven, Belgium.
2
Department of Geosciences, University of Nevada Las Vegas, 4505 S Maryland Parkway, Las
Vegas, NV 89154-4010, USA.
2
Dirk Goossens and Brenda Buck
2008 [3]. In Montana (USA) the number of registered ORV drivers doubled between 2002
and 2007 [4]. In southern Nevada (USA) the number of off-road drivers has quadrupled in
only the last few years [5]. In 2008, the Bureau of Land Management in Las Vegas
estimated that the number of off-road drivers in the city (residents plus tourists) had
increased to more than 300 000 [6]. Elsewhere in the world ORV activity is also increasing,
on all continents [7].
Emissions of soil dust created by ORV activity were hardly studied before the early 1990s,
but received much attention since then: see [8], [9], [10], [11], [12], to cite only a few
studies. The importance of ORV driving on dust production, at least on a local scale, is very
well illustrated by the study by Goossens et al. [13], who found that the contribution of
ORV to dust production can be of the same order as that generated by wind, even in areas
prone to severe wind erosion.
Most studies dealing with ORV dust focus on the emission rates, using active or passive
sampling techniques. Surprisingly little attention has been paid to the characteristics of the
dust clouds themselves, to their dynamics, and to the implications for health for the drivers
and the surrounding passive recreants. For example, Wheeler [14] measured dust
concentrations with a real-time dust monitor on a trailing vehicle following an ORV, but
did not provide information on temporal or spatial variations of the concentration within the
cloud. USEPA [15], with a summary in Lytle and Woo [16] performed experiments with
(among others) ORVs to measure exposure to asbestos from natural soils in Clear Creek
Management Area, California, USA. Motorcycle riding, quad riding and SUV (Sports
Utility Vehicle) driving were compared, but no analysis was made of the dust clouds
produced by them. Padgett et al. [17] performed samplings of dust generated by ORVs at
Turkey Bay (Kentucky, USA), but did not characterize the structure of the clouds although
they pointed out that concentration varied between locations and, thus, measurements taken
at a single point are not appropriate for estimating erosion losses. Goossens and Buck [12]
measured dust emissions for 3 types of ORVs (motorcycles, dune buggies, quads), 17
different types of surfaces, and 3 driving speeds, but did not provide information on
temporal or spatial variations of the concentrations or on the structure of the dust clouds
produced. Jia et al. [18],[19] collected dust during experiments with a car on two unpaved
roads in Sweden, but no analysis was made of temporal or spatial variations of
concentration in the cloud.
Real-time monitoring systems were developed to measure dust concentrations in front and
behind a vehicle, such as TRAKER (Testing Re-entrainment Aerosol Kinetic Emissions
from Roads, see [20], or SCAMPER (System of Continuous Aerosol Monitoring of
Particulate Emissions from Roadways, see [21], but these do not measure the temporal or
spatial evolution of the concentration in the cloud because they follow the vehicle as it
moves along it pathway.
Several attempts were made to model the emissions. For example, Thornburg et al. [22]
developed a probabilistic model to calculate the concentration in the trailing wake of a dust
source. Parameters considered were: emission rate of the source, velocity (relative to the
source) of the measuring point, distance from the source, offset angle from the centerline of
movement, and turbulent diffusivity. Their model also contains a module for a quiescent
environment scenario. Comparisons were made with experimental data from seven earlier
conducted studies, but no information was given on the internal structure of the dust clouds,
on temporal dilution patterns, or on total dust concentrations and their variation within the
cloud. Jazcilevich et al. [23] developed a stochastic model based on empirical probability
distribution functions to estimate human exposure to emissions from the resuspension of
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
3
road dust due to isolated wakes from moving vehicles. They describe the pattern of primary
and secondary concentration waves in a dust cloud generated by a vehicle, but did not
quantify this pattern in their field experiments, which mainly focused on the statistical
distribution of exposure classes within the dust cloud.
The work by Mao et al. [24] is one of the rare studies that measured the temporal pattern of
concentration in a dust cloud produced by a vehicle. Concentrations were measured with
automated particle counters, for a number of vehicle passings, and concentrations were
plotted as a function of time so that the changes in concentration could be followed in detail.
However, measurements were performed on a single surface only (a dirt gravel road); all
measurements were made well off the road, at distances of 67.5 m, 110 m and 160 m from
the road, and the vehicle was a ¾ ton truck (not an ORV). Although useful for comparisons
with the current study, no attempts were made to calculate concentration on the road itself,
for different driving speeds, different types of surfaces, or to present the dilution factor in
terms of distance traveled along the road. Temporal patterns of dust concentration in a
direction perpendicular to the road were also measured by [25], [26], [27] and [28], but
these studies do not discuss the temporal and spatial patterns behind the vehicle, or the
structure of the dust cloud.
Despite the important progress made during especially the last decade, little is thus known
as yet regarding the structure of an ORV-produced dust cloud, its characteristics, and its
evolution in time and space once the vehicle has passed. Such information is important,
however, because ORV driving often occurs as a social activity in a group (Figure 1), with
the riders exposed to dust produced by vehicles driving in front of them. Passive visitors
near the road are also exposed to the dust. What are the peak concentrations in the dust
clouds, and how do they relate to the average concentrations? How fast (or how slowly)
does dilution take place? How long does it take before the cloud has disappeared and
concentrations have returned to the original background levels? How distant from an ORV
should one ride to reduce exposure to levels safe for human health? How does all this
information change as a function of ORV type, driving speed and terrain type? How does
wind direction affect the data? Despite their importance, these questions currently remain
unanswered.
The aim of this article is to provide more details on the characteristics and dynamics of dust
clouds produced by ORVs. Field experiments were conducted for that purpose in the Nellis
Dunes Recreational Area (NDRA), one of the most popular off-road driving areas in
southern Nevada, USA. According to a Bureau of Land Management study [29], 285 000
people visit the NDRA annually and the population exposed to the dust is therefore huge. In
addition, the NDRA is characterized by many different terrain types, varying from sand
dunes, silt surfaces, rock-covered surfaces, to different types of drainages. Therefore, it is
an ideal place to study dust production by ORV activity.
2 Study Area and Selected Surfaces
Nellis Dunes Recreational Area is located some 6 km from the northeastern margin of the
conurbation of Las Vegas, North Las Vegas and Henderson, Nevada, USA (Figure 2). It
encompasses a terrain of approximately 36 km2. More than forty years of unlimited
off-road driving have resulted in the creation of thousands of road tracks in the desert floor,
with a total combined length of 533 km [30].
For the current study we selected 7 different types of surfaces, representative for the NDRA
4
Dirk Goossens and Brenda Buck
and for off-road terrain in general. Special attention was paid to ensure that coarse-grained,
medium-grained and fine-grained substrata were tested, as grain size is one of the dominant
parameters affecting the intensity of dust emissions from ORV driving [31],[32]. The 7
surface types were as follows:
• surface 1: active sand dunes, devoid of any vegetation
• surface 2: partially vegetated sand dunes
• surface 3: slightly silty sand, without vegetation
• surface 4: sandy drainage
• surface 5: shallow (usually <5 cm) silty layer on limestone bedrock
• surface 6: aggregated silt
• surface 7: fine-grained silt, not aggregated
The grain size characteristic of these surfaces (obtained by dry sieving of the <2 mm
fraction) are shown in Figure 3. More details on rock fragment cover, surface crusting and
vegetation are given in Table 1.
Figure 1: Off-road driving in echelon. Note driving in dense dust clouds.
3 Experimental Procedure
A 750 Kawasaki Brute Force quad with standard off-road tires was used during the
experiments. Riders were asked to drive on a more or less straight road segment
representative for the type of surface tested. Three speeds were selected: 24 km h-1 (or 15
mph), 32 km h-1 (or 20 mph), and 40 km h-1 (or 25 mph). No higher speeds were tested for
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
5
safety reasons, but 40 km h-1 was close to the maximum speed a quad could drive on the
experimental terrain. Between 10 and 20 repetitions were performed for each driving speed.
A DustTrak DRX 8533 real-time aerosol monitor (TSI Inc., Shoreview, MN, USA) was
used to measure the concentrations in the dust cloud produced by the vehicle. For this study
we analyzed the PM10 fraction (particles < 10 μm, which are the most easily inhaled); all
concentrations presented in this article are thus for PM10. Concentrations were measured at
about 130 cm above the ground, which corresponds closely to nose level of most riders
driving a quad.
115°30'W
115°00'W
I-15
US 95
36°30'N
Nellis
Dunes
36°30'N
US 93
NORTH LAS VEGAS
LAS VEGAS
Lake Mead
36°00'N
36°00'N
HENDERSON
OREGON
BOULDER CITY
IDAHO
NEVADA
NEVADA
UTAH
CALIFORNIA
N
I-15
ARIZONA
115°30'W
0
10
115°00'W
Figure 2: Location of Nellis Dunes Recreational Area
20km
6
Dirk Goossens and Brenda Buck
Table 1: Characteristics of the 7 surfaces investigated in this study. For information on
grain size characteristics see Figure 3.
100
90
dust surfaces
sand surfaces
cumulative percent (%)
80
70
60
50
40
shallow silt on limestone
non-aggregated silt/clay
30
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
20
10
sandy drainage
0
10
100
1000
10000
particle diameter (µm)
Figure 3: Grain size curves for the 7 surfaces tested in this study. Data refer to the <2 mm
fraction and were obtained by standard dry sieving
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
7
Before each run, the background PM10 concentration was measured during at least 30
seconds. Runs only took place when the background was very low, less than 0.03 mg m-3,
and remained constant over time. After 30 seconds the vehicle performed its run, making
sure that the target speed was attained well before the measuring point was reached.
Immediately after the vehicle's passage the DustTrak was brought into the road's centerline
(roads were usually less than 2 m wide), at a height of 130 cm, and PM10 concentration was
measured for at least 90 s, or longer if the background concentration was not yet reached
after this time. Concentrations were measured every second, and stored with the same
frequency.
Zeroing of the DustTrak was carried out at regular intervals to ensure that the recorded
concentrations were correct at all times.
Experiments were performed at episodes when the wind speed was less than 1 m s-1. After
completion of the experiments, all data were downloaded to a computer. Results were
plotted on a graph for each experiment (given combination of driving speed and surface
type), and runs that had been disturbed by wind gusts were removed from the data set. All
data presented in this article thus refer to "undisturbed", calm conditions. However, in
section 4.5 we also present some data obtained during periods of headwind and tailwind
and discuss how concentrations change during these and oblique winds.
Results (concentration as a function of time) were checked for each run, and the average of
all selected runs was then calculated for each combination of terrain type and driving speed.
Differences between homologous runs were usually small to very small due to the
comparable conditions between runs
4 Results and Discussion
The PM10 concentration curves are shown in Figure 4, for all 7 surface types tested. Each
diagram shows the results for the 3 driving speeds selected. Note the differences in the
vertical scale between the diagrams.
8
Dirk Goossens and Brenda Buck
120
PM10 concentration (mg m )
20
15
10
24 km/h
32 km/h
40 km/h
5
shallow silt on limestone
-3
dunes without vegetation
-3
PM10 concentration (mg m )
25
0
100
80
60
40
24 km/h
32 km/h
40 km/h
20
0
0
20
40
60
80
100
0
time after vehicle passage (s)
60
80
100
140
PM10 concentration (mg m )
vegetated dunes
40
non-aggregated silt/clay
-3
-3
PM10 concentration (mg m )
40
time after vehicle passage (s)
45
35
30
25
20
15
24 km/h
32 km/h
40 km/h
10
5
120
100
80
60
40
24 km/h
32 km/h
40 km/h
20
0
0
0
20
40
60
80
0
100
20
40
60
80
100
time after vehicle passage (s)
time after vehicle passage (s)
35
PM10 concentration (mg m )
70
30
aggregated silt
-3
slightly silty sand
-3
PM10 concentration (mg m )
20
25
20
15
10
24 km/h
32 km/h
40 km/h
5
60
50
40
30
20
24 km/h
32 km/h
40 km/h
10
0
0
0
20
40
60
100
80
0
20
40
60
80
time after vehicle passage (s)
time after vehicle passage (s)
-3
PM10 concentration (mg m )
25
sandy drainage
20
15
10
24 km/h
32 km/h
40 km/h
5
0
0
20
40
60
80
100
time after vehicle passage (s)
Figure 4: PM10 concentration on the road at 130 cm height, for the 7 tested surfaces. Left:
sandy surfaces; right: silty surfaces. Note the differences in the vertical scale
100
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
9
4.1 Structure of an ORV generated Dust Cloud
The PM10 concentration at 130 cm height varies considerably within an ORV cloud
(Figure 4). Maximum concentration is reached quickly, followed by a more or less
asymptotical decrease towards the background value. Irregularities in the curve occur at
any time, but are most pronounced after the maximum.
Jazcilevich et al. [23] published a scheme for the dust concentration in a cloud produced by
a road truck (see Figure 5A). Although ORVs are somewhat different in size and weight
compared to trucks, Jazcilevich et al.'s scheme resembles the curves in Figure 4 and helps to
explain what is happening. According to Jazcilevich et al., a main wave of dust is followed
by secondary fronts of lesser amplitude. The first wave is stronger, since it is part of the first
swirl of a screw-like wave generated on each side at the rear of the vehicle. Secondary
fronts, on the other hand, are part of swirls already attenuated by contact with the ground.
Although the pattern generated by an ORV is similar, it can be described in more detail (see
also Figure 5B). Dust is released from the ground by two mechanisms: direct uplift due to
the contact between the wheels and the surface, and indirect uplift caused by turbulence
generated by the vehicle. Pulverization of the top layer by the tires facilitates the emission
as it breaks any coarse aggregates and any surface crusts (if present) when the tires drive
over them [33]. After having been released, the dust is dispersed in three directions:
longitudinally (either forward or backward), laterally, and vertically. Coarse, sand-sized
particles will fall back to the surface almost immediately. Finer particles (dust), which are
transported in suspension, are affected by the turbulent eddies created by the vehicle and
form the largest (and highest) part of the cloud. Because it takes awhile for the lifted
particles to get dispersed, concentration at drivers' nose level will be very low immediately
behind the vehicle (see Figure 5B). As time elapses, the turbulent eddies expand and the
lobes in the dust cloud become better visible (Figure 5B). Maximum concentration is
reached after approximately 5-7 seconds; this corresponds to the peak in the concentration
curves in Figure 4. The variations in concentration that occur after the maximum
(Jazcilevich et al.'s "secondary waves") are caused by these turbulent eddies, as can be seen
right in Figure 5B. Note that the "secondary waves" can be very pronounced (Figure 4E) or
almost absent (Figure 4A).
The pattern described above is typical for calm (very light wind to no-wind) conditions.
During headwind or tailwind, some differences may occur (see section 4.5).
This article focuses on the dust concentration and its temporal and spatial variation above
the road. No analysis is made of the temporal and/or spatial variations that occur in the
lateral direction; information on these topics can be found in [25], [26], [27] and [28].
4.2 Maximum (peak) Concentration
Because the airborne concentration in an ORV cloud can be very high (Figure 4), it is
important to check the maximum values because these occur at distances of the order of a
few tens of meters behind the vehicle, i.e., the zone in which trailing drivers usually ride
when driving in echelon (Figure 1). Maximum concentrations are plotted in Figure 6A; the
time (after passage of the vehicle) at which they were reached is shown in Figure 6B.
Maximum PM10 concentration increases with the driving speed, and for 6 of the 7 tested
surfaces the increase was exponential (Figure 6A). For the non-aggregated silt the increase
was almost linear. These findings are consistent with what was reported in earlier studies,
which also found either linear ([10],[12],[34]) or exponential ([12],[26],[34]) relationships.
10
Dirk Goossens and Brenda Buck
Also, note that the maximum PM10 concentrations are generally higher for silty surfaces
than for sandy surfaces, as could be expected, but the difference is more pronounced when
the driving speed is high, about 30 km h-1 or more.
A
B
Figure 5: Structure of an ORV cloud. A: Schematic representation of the waves in a
resuspension cloud. Reprinted from Aeolian Research, vol. 4, Jazcilevich, A., Wellens, A.,
Siebe, C., Rosas, I., Bornstein, R.D., Riojas-Rodríguez, H., Application of a stochastic
vehicular wake erosion model to determine PM2.5 exposure, pp. 31-37, Copyright (2012),
with permission from Elsevier; B: Development of a resuspension cloud behind a quad
Time (after the vehicle's passage) after which the maximum concentration was reached
varied between 5 and 9 s (Figure 6B); in most of the experiments it was between 5 and 7 s
(average for all experiments: 6.2 s). There was no relation with the driving speed, and also
no unequivocal relation with the type of surface (sandy or silty topsoil) although for two of
the three silty surfaces it took more than 6.5 s to attain the maximum concentration
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
11
4.3 Dilution
Dilution of the dust cloud was checked using two criteria: (1) time (after the vehicle's
passage) it took for the concentration to return to the original background level, and (2) the
speed of decrease of the concentration. The latter criterion quantifies the shape of the
concentration curves in Figure 4 after the maximum
-3
maximum concentration (mg m )
140
shallow silt on limestone
non-aggregated silt/clay
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
sandy drainage
120
100
80
A
60
40
20
0
0
10
20
30
40
50
driving speed (km h-1)
time after vehicle passage (s)
18
shallow silt on limestone
non-aggregated silt/clay
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
sandy drainage
16
14
12
B
10
8
6
4
2
0
20
25
30
35
40
45
-1
driving speed (km h )
Figure 6: A: Maximum concentration as a function of driving speed; B: Time (after passage
of the vehicle) when the maximum concentration was reached.
12
Dirk Goossens and Brenda Buck
Elevated PM10 concentrations (defined here as concentrations above the background) were
measured for approximately 1 minute after the passage of an ORV (Figure 7A). Even when
not visually noticeable, fine particles will thus remain suspended for quite awhile at
elevations where they can be inhaled. Therefore, ORV driving poses possible concerns for
health even when the atmosphere is visually "clean". No relationship was observed between
the duration of the elevated concentrations and driving speed except for the finest soil
(non-aggregated silt). There was also no systematic difference between the silty and the
sandy soils.
160
shallow silt on limestone
non-aggregated silt/clay
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
sandy drainage
lifetime of cloud (s)
140
120
100
A
80
60
40
20
0
20
25
30
35
40
45
-1
driving speed (km h )
0.02
shallow silt on limestone
non-aggregated silt/clay
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
sandy drainage
0.00
slope parameter b
-0.02
-0.04
B
-0.06
-0.08
-0.10
-0.12
-0.14
-0.16
20
25
30
35
40
45
-1
driving speed (km h )
Figure 7: A: Lifetime (on the road) of an ORV produced dust cloud; B: Rate of decrease of
PM10 concentration in an ORV produced dust cloud; ordinate shows parameter b of the
function C = e ( a +bT ) , where C = PM10 concentration (mg m-3) and T = time (s).
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
13
The speed of decreases (after maximum concentration has been reached) can be accurately
described by an exponential function C = e ( a +bT ) , where C is concentration, T is time (after
the maximum), and a and b are empirical parameters controlled by the driving speed, the
type of vehicle and the characteristics of the surface. Parameter b quantifies the rate of
decrease in the curves in Figure 4. Other studies (e.g. [24],[25],[26],[27],[28]) also found
exponential-like decreases of concentration with time, at least sufficiently close to the road;
at larger distances the decrease occurred much more gradually (see [24] and [27]). As can
be seen in Figure 7B the rate of decrease in concentration is not related to the driving speed
or, somewhat surprising, the type of surface (silty or sandy soil). These finding concur with
those observed in Figure 7A and show that dilution of an ORV generated dust cloud is not
necessarily related to driving speed or type of surface.
An interesting, and also important consequence is that in-echelon drivers should keep large
distances between their vehicles if they want to avoid considerable exposure to dust. In
Figure 8 we plotted the distance within which elevated PM10 concentrations occur behind
an ORV. For driving speeds as low as 24 km h-1 the critical distance is already around 400
m, and for a driving speed of 40 km h-1 it is between 600 and 800 m or even more. We
acknowledge that these values will be considerably lower if there is wind, especially when
blowing obliquely to the driving path, but the values shown in Figure 8 will not be far off
what happens in wind sheltered zones such as little canyons or wind protected leesides of
small and larger hills. Also, one should realize that for most of the cases tested in Figure 4
concentration had already decreased to less than 5 mg m-3 after about 20 seconds or shorter.
Even for this value, the distance required to obtain it is between 100 and 200 m for common
driving speeds (see the 20-second line in Figure 8), which is much more than what is
commonly happening during normal ORV recreation (Figure 1). Driving in echelon on
unpaved terrain is thus strongly dissuaded unless protective equipment such as a dust mask
is worn. An alternative is to drive sufficiently close behind the lead vehicle and stay within
the low concentration zone (at least, at nose level) immediately behind it, but this implies
following the vehicle as close as only a few meters and is thus not really recommended in
terms of safety.
14
Dirk Goossens and Brenda Buck
1400
shallow silt on limestone
non-aggregated silt/clay
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
sandy drainage
after 20 seconds
1200
distance (m)
1000
800
600
400
200
0
0
10
20
30
40
50
-1
driving speed (km h )
Figure 8: Length of zone behind an ORV where elevated PM10 concentrations occur. Data
are shown for all 3 driving speeds and 7 types of surfaces tested. The 20-second line
indicates the distance traveled after 20 seconds
Our results do not support the opinion of Jazcilevich et al. [23], who state that the effects of
vehicular resuspended dust do not reach more than 50 m from the source. In fact, Mao et al.
[24] measured elevated concentrations up to 160 m from the road. On the road itself, and
under calm conditions, the numbers can be even much higher, as demonstrated by our
study.
4.4 Dosage
The amount of dust in an ORV cloud at riders' nose level (130 cm in this study) can be
estimated by multiplying the average concentration, at 130 cm, by the lifetime of the cloud.
This very closely approaches the time integrated area under the ensemble mean
concentration curves in Figure 4. Mao et al. [24] called this the "pulse area", or dosage. In
their experiments they found values of the order of 10 mg m-3 s per passage (of a ¾ ton
truck), but these were measured at distances between 67.5 and 160 m from the road. The
dosages in our ORV experiments are much higher, up to 1294 mg m-3 s for the silty surface
on top of limestone (Figure 9). The figure also shows that dosage increases exponentially
with driving speed, and that silty surfaces produce much more dust than sandy surfaces, as
expected.
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
15
-3
dust concentration x time (mg m s)
1400
shallow silt on limestone
non-aggregated silt/clay
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
sandy drainage
1200
1000
800
600
400
200
0
0
10
20
30
40
50
-1
driving speed (km h )
Figure 9: Dosage (product of average PM10 concentration and lifetime of the cloud) for the
3 driving speeds and 7 types of surfaces tested
Another way to quantify the effect an ORV has on dust concentration is to calculate how
much more dust is present in the air (in our case: at 130 cm) compared to undisturbed
conditions. Results are shown in Figure 10. Even at the lowest driving speed tested, 24
km h-1, the concentration (measured over the entire lifetime of the cloud, i.e., not just at or
near peak concentration time) is dozens of times higher than the background concentration.
The effect of ORV driving on airborne dust concentration is thus considerable
increase in dust concentration
600
shallow silt on limestone
non-aggregated silt/clay
aggregated silt
dunes without vegetation
vegetated dunes
slightly silty sand
sandy drainage
500
400
300
200
100
0
0
10
20
30
40
50
-1
driving speed (km h )
Figure 10: Average (over lifetime of the cloud) PM10 concentration relative to background
PM10 concentration
16
Dirk Goossens and Brenda Buck
Somewhat surprisingly, and in contrast to the dosage data in Figure 9, no relationship was
found with the type of surface (silty versus sandy topsoil) in Figure 10 despite the high
concentrations for the shallow silt on limestone.
4.5 Wind Effects
The data and conclusions in the previous sections were derived for calm conditions, with no
or only very light winds. In this section we investigate what happens to the dust
concentration when there is wind.
First, it is important to realize that even under no-wind conditions a vehicle creates a zone
of moving air as it advances over the road. Air in front of the vehicle is pushed forward, and
air in the low pressure zones aside and above the vehicle and in the vehicle's wake is pulled
in the direction of movement. This means that even in calm ambient conditions, some of the
dust emitted before the vehicle reaches the measuring point will pass through the measuring
point and contribute to the concentration measured at it.
If there is ambient wind, the following situations may occur:
•
•
•
The wind blows obliquely to the driving path (road). This results in the dust drifting
away from the road, and dust concentration above the road decreases rapidly. Trailing
vehicles will remain (or, depending on the wind speed, largely remain) out of the cloud
created by a lead vehicle.
Tailwind conditions. Part of the dust produced upstream of the measuring point is
blown extra fast (faster than caused by the moving vehicle) through the measuring point.
Dilution of the dust cloud near the measuring point occurs faster than normal; dust
concentration decreases faster than under no-wind conditions.
Headwind conditions. If this happens, part of the dust emitted down the measuring
point is blown in the opposite direction, towards the measuring point. Depending on the
strength of the suction force created by the moving vehicle (driving speed is an
important factor here), this dust will either be effectively transported towards the
measuring point or move slower than normal away from it. In both cases dust
concentration at the measuring point will decrease at a slower rate compared to no-wind
conditions.
For a person standing in the driving route of an ORV after the vehicle has passed, the order
of the risk to inhale large amounts of dust emitted by the ORV is thus as follows: headwind
(highest risk), no wind, tailwind, oblique wind (lowest risk). For a rider in an ORV
following a lead vehicle, and who maintains sufficient distance from the lead vehicle (see
the previous sections), the same order applies.
The effect of headwind and tailwind is illustrated in Figure 11. To adequately compare the
patterns the data were normalized in each diagram. The faster dilution during tailwind and
the slower dilution during headwind compared to calm conditions are clearly visible in the
curves. Also note that the irregularities in the curves (Jazcilevich et al.'s [23] "secondary
waves") are more pronounced during headwind conditions than during calm and tailwind
conditions, which is a logical consequence of the much more complex and chaotic
movement of air and dust during the former compared to the latter.
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
17
5 Conclusions
ORV riding can produce dust clouds with PM10 concentrations up to 160 mg m-3 and more
at nose level of drivers or passive recreants standing near an ORV trail. Total suspendable
particle (TSP) concentrations are even higher, but were not measured in this study.
ORV clouds may show slight differences according to wind conditions, but are generally
characterized by a main wave of high dust concentration followed by an exponential (in
time and space) decrease in concentration until the original background level has again
been reached. The decrease is characterized by irregularities ("secondary waves") that are
especially visible during headwind conditions. Tailwind and no-wind conditions produce
more regular dust concentration curves.
Peak (maximum) concentration at nose level in an ORV cloud is reached shortly after the
passage of the vehicle, usually after about 5-7 seconds, and peaks become higher as the
driving speed increases. Fine-grained, silty surfaces also result in higher peak
concentrations than more coarse-grained, sandy soils.
Under no-wind and headwind conditions it takes at least a full minute after the passage of
an ORV before PM10 concentration at nose level has returned to the original background
level. During tailwind the period of elevated concentration is shorter, but still of the order of
30 seconds or more. At winds blowing obliquely to the driving path it is only a few seconds.
No relationship was observed between the duration of elevated concentration and driving
speed, except for the finest soil, which consisted of fine, non-aggregated silt.
For driving speeds between 24 and 40 km h-1, elevated dust concentrations may occur up to
several hundreds of meters behind an ORV vehicle, even if no dust is visually noticeable.
Driving in echelon behind a lead vehicle is thus not recommended unless appropriate
protection is worn against inhaling the dust.
18
Dirk Goossens and Brenda Buck
normalized dust concentration
1.0
no wind
0.9
A
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80
time (s)
normalized dust concentration
1.0
tailwind
0.9
B
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80
time (s)
normalized dust concentration
1.0
headwind
0.9
C
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80
time (s)
Figure 11: Typical dust concentration curves for (A) no-wind conditions; (B) tailwind
conditions; (C) headwind conditions. Curves were normalized to facilitate comparison. A:
sandy drainage, 32 km h-1; B and C: slightly silty sand, 32 km h-1.
Dynamics of Dust Clouds Produced by Off-road Vehicle Driving
19
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